The pressurised gas supplied in CPAP treatment of OSA serves to pneumatically splint open the patient's airways. The pressure of the supplied gas may be constant, bi-level (in synchronism with patient breathing) or auto-setting in level. Throughout this specification any reference to CPAP is intended to incorporate a reference to any one of, or combinations of, these forms of breathable gas supply.
CPAP treatment is generally administered whilst the patient and any bed partner are sleeping. As the gas supply apparatus is normally located within a few meters of the patient it is desirable to minimise the noise produced by that apparatus to minimise sleep disturbance.
CPAP breathable gas supply apparatus generally comprise a plastics housing or casing having a gas flow generator assembly and an electrical control and power supply system therein. A flexible conduit connects the outlet of the apparatus (at a point on the housing) to a nose and/or mouth mask worn by the patient to communicate the supplied gas to the patient's airways.
The flow generator assembly usually consists of a brushless electric motor driving a fan or turbine. The noise produced by the flow generator assembly has three basic transmission paths to surrounding atmosphere. It is radiated from the apparatus housing, transmitted from the turbine outlet to be propagated along the conduit that connects the outlet of the apparatus to the patient mask and transmitted from the turbine inlet to be propagated along the gas inlet path (in the opposite direction to the gas flow) to the housing gas inlet and so to atmosphere.
In a prior art approach, the flow generator assembly has been mounted to the apparatus housing by fastening the turbine housing to mounting feet integrally moulded with the apparatus casing with cushioning rubber washers disposed between the housing and feet. In addition to having acoustic air paths for noise emanating from the flow generator through the conduit and housing, vibration produced by the flow generator assembly is transmitted through the feet to the housing, which acts as a panel radiator, and radiates noise therefrom. The vibration energy reaching the casing can also result in a buzzing noise or the like that can be particularly disturbing to the patient and any bed partner.
An attempt to reduce noise radiated from the housing has involved attaching numerous, for example about ten, blocks of acoustically absorptive foam to the inner surfaces of the apparatus housing. However, this increases the complexity, and thereby the cost, of assembling the apparatus.
Another approach is used in the applicant's bi-level CPAP apparatus model VPAPII. The VPAPII includes a sound enclosure within the apparatus housing having an outlet chamber mounted therein. The metal sound enclosure has a first and second chamber, each having a port to allow the passage of air into the first chamber through to, and then out of, the second chamber. The flow generator assembly and the outlet chamber are located in the second chamber. Air is drawn past a baffle and into the first chamber which includes a step-like labyrinth baffle allowing the free flow of air through the first chamber into the second chamber whilst attenuating the noise from the flow generator assembly propagating along the air inlet path. The internal surfaces of the sound enclosure are lined with sound absorbing polyurethane skinned foam.
In one version of VPAPII, the flow generator assembly is mounted within the sound enclosure and attached to the outlet chamber by a rigid metal mounting bracket. In another earlier version, the flow generator assembly sat on one inner face of the second chamber and was cushioned by EVA foam that was in turn adhered to the second chamber inner face. In both versions, the blower air path outlet is secured to the inlet port of the outlet chamber by way of a silicone rubber conduit.
The outlet chamber is formed as one substantially rectangular chamber moulded from "ignition resistant" ABS. Foam is adhered to the outlet chamber's internal surfaces but otherwise the outlet chamber is `empty` in that it has no labyrinthine or tortuous path.
The VPAPII, whilst being quieter than previous apparatus, requires expensive materials to produce, is complex in both manufacture and assembly and does not allow for rapid reassembly after servicing. It also utilises steel components which are relatively heavy and affect the portability of the apparatus.
It is an object of the present invention to substantially overcome or at least ameliorate one or more of the deficiencies of the prior art.